Electronic Transition Moments of 1,3,2-Benzodiazaboroline (‘External’ Bn Indole) and ‘Fused’ Bn Indole, Containing the 1,2-Dihydro-1,2-Azaborine Core

Abstract

The biological importance of tryptophan as an amino acid, an essential metabolite, and a precursor to biosynthetic pathways cannot be overstated. As such, the photo-physical properties of tryptophan are of great interest for spectroscopic studies to probe the local interactions between bio-molecular residues in proteins and macromolecular complexes. Although tryptophan is intrinsically fluorescent, it is often difficult to isolate its signal from background and scattered excitation light. Here we investigate the electronic structure of a tryptophan analogue with distinct optical properties that can potentially be used as a tryptophan substitute in many biological contexts. Boron-Nitrogen/Carbon=Carbon isosterism of indole-based structures provide a class of boron containing indole derivatives, which exhibit distinct electronic characteristics. We studied the orientations and magnitudes of the electric dipole transition moments (EDTMs) of 1,3,2-benzodiazaboroline (‘external’ BN indole) and 'fused' BN indole, containing the 1,2-dihydro-1,2-azaborine core. A combination of absorbance spectroscopy, ultraviolet linear dichroism (UV-LD), infrared linear dichroism (IR-LD) and quantum chemical calculations provided the means to assign two EDTMs for the external BN, and two EDTMs for the fused BN indole within the 30,000-70,000 cm-1 spectral range. We constructed an excited state model for each molecule utilizing Density-Functional Theory for the lowest ground state equilibrium geometry with a single point first-excited state, using BL3YP and ωB79X. The theoretical predictions for low-energy transition orientations are in good agreement with our experiment data.